Editing /dev/random (section)

{{Short description|Pseudorandom number generator file in Unix-like operating systems}}
[[File:Hexdump of dev urandom.png|thumb|upright=1.5|256 byte hex dump of {{nowrap|{{mono|/dev/urandom}}}}]]
In [[Unix-like]] [[operating system]]s, '''{{nowrap|{{mono|/dev/random}}}}''' and '''{{nowrap|{{mono|/dev/urandom}}}}''' are [[special file]]s that provide random numbers from a [[cryptographically secure pseudorandom number generator]] (CSPRNG). The CSPRNG is seeded with [[Entropy_(computing)|entropy]] (a value that provides [[randomness]]) from environmental noise, collected from [[device driver]]s and other sources. Users can obtain random numbers from the CSPRNG simply by reading the file.<ref>{{cite web
| url=https://man7.org/linux/man-pages/man7/random.7.html
| title=random(7) - Linux manual page
| date=2023-02-10
| access-date=2023-11-24
}}</ref> Not all operating systems implement the same methods for {{nowrap|{{mono|/dev/random}}}} and {{nowrap|{{mono|/dev/urandom}}}}.

In older operating systems, {{nowrap|{{mono|/dev/random}}}} typically [[Blocking (computing)|blocked]] if there was less [[Entropy (computing)|entropy]] available than requested; more recently (see below for the differences between operating systems) it usually blocks at startup until sufficient entropy has been gathered, then unblocks permanently. The {{nowrap|{{mono|/dev/urandom}}}} device typically was never a blocking device, even if the pseudorandom number generator seed was not fully initialized with entropy since boot.

This special file originated in Linux in 1994. It was quickly adopted by other Unix-like operating systems.<ref name=":0" />